With many engines it is desirable to have both a positive power mode of operation (in which the engine produces power for such purposes as propelling an associated vehicle) and a braking mode operation (in which the engine absorbs power for such purposes as slowing down an associated vehicle). It is well known that a highly effective way of operating an engine in braking mode is to cut off the fuel supply to the engine and to then open the exhaust valves in the engine near top dead center of the compression strokes of the engine cylinders. This allows air that the engine has compressed in its cylinders to escape to the exhaust system of the engine before the engine can recover the work of compressing the air during the subsequent "power" strokes of the engine pistons. This type of engine braking is known as compression release engine braking.
It takes a great deal more force to open an exhaust valve to produce a compression release event during compression release engine braking than to open either an intake or exhaust valve during positive power mode operation of the engine. During positive power mode operation the intake valves typically open while the piston is moving away from the valves, thereby creating a low pressure condition in the engine cylinder. Thus the only real resistance to intake valve opening is the force of the intake valve return spring which normally holds the intake valve closed. Similarly, during positive power mode operation the exhaust valves typically open near the end of the power strokes of the associated piston after as much work as possible has been extracted from the combustion products in the cylinder. The piston is again moving away from the valves and the cylinder pressure against which the exhaust valves must be opened is again relatively low. (Once opened, the exhaust valves are typically held open throughout the subsequent exhaust stroke of the associated piston, but this only requires enough force to overcome the exhaust valve return spring force.)
Four cycle internal combustion engines, conventionally, are outfitted with either mechanical or hydro-mechanical intake and exhaust opening systems. These systems may include a combination of camshafts, rocker arms and push rods that operate synchronously with the engine's crankshaft rotation. The timing of the valve openings is fixed in relationship to the position of the crankshaft by direct mechanical connection of the valve actuating system with the crankshaft. In any cylinder, of a multi-cylinder internal combustion engine, intake and exhaust valve openings and closings in conjunction with the fuel mixture and either ignition or fuel injection, are predetermined to provide optimum positive power over a range of engine speeds. This relationships between the piston motion of a cylinder and its intake and exhaust valve openings and closings, for a conventional internal combustion engine is illustrated in FIG. 1.
The crankshaft of a four-cycle internal combustion engine rotates through 720.degree. during one series of its four strokes (i.e., compression, expansion, exhaust and intake). FIG. 1 depicts the relationships between the piston and valves beginning with the piston at top dead center ("TDC") of the compression stroke 5. Both the intake and exhaust valves are closed, and remain closed during most of the expansion stroke wherein the piston is traveling away from the cylinder head (i.e., the volume between the cylinder head and the piston head is increasing). Fuel is burned during the expansion stroke and positive power is delivered by the engine. As the piston reverses direction at the end of the expansion stroke, the exhaust valve opens, illustrated as 7 in FIG. 1 and combustion gases are forced out of the cylinder as the piston travels again to exhaust TDC 6. Just prior to the exhaust TDC, the intake valve opens, illustrated as 8 in FIG. 1. Immediately after the exhaust TDC, the exhaust valve closes, and air or fuel mixture is drawn into the cylinder chamber through the intake valve as the piston travels away from the cylinder head. The intake valve closes when the piston is near the or in the proximity of the furthest distance from the cylinder head. Subsequently, both the intake and exhaust valves are closed, and the compression stroke begins bringing the piston to TDC and the four cycle repeats.
FIG. 2 illustrates the required intake and exhaust valve openings that occur when an internal combustion engine operates in a braking mode (i.e., as a compressor wherein the compressed air is evacuated at the vicinity of TDC compression). FIG. 2 also illustrates engine piston motion. During the braking mode, no fuel is being supplied to the engine. As a result, only air is being compressed during the compression stroke. FIG. 2 depicts the normal intake and exhaust valve openings (i.e., during positive power) during the exhaust and intake strokes of the piston. Additionally, an exhaust valve opening 9 is shown immediately before the completion of the compression stroke and subsequent to the closing prior to the beginning of the exhaust stroke. There are other options. This is just one example of an exhaust cam operated compression release brake. Engine braking is achieved during the compression stroke and the evacuation, by way of the added exhaust valve opening, of the compressed air immediately following.
The aforementioned process described compression release engine braking. The additional exhaust valve opening is achieved by adding components that actuate an exhaust valve independently from the normal actuating mechanisms. This is typically achieved by actuating the lifting mechanism of the exhaust valve by way of a secondary hydro-mechanical system that can be deactivated when the engine is operating in its positive power mode. In summary, the secondary system lifts the exhaust valve, at an appropriate time, and does not interfere with, nor interrupt, the normal valve lifting mechanism, and is inactive during positive power operation. Timing of the secondary system's valve lifting is usually derived from the activation of an adjacent cylinder's normal intake or exhaust valve's opening or the injection actuation mechanism. A neighboring cylinder, wherein a valve opening occurs nearest to the desired time for the active cylinder's exhaust valve opening is chosen. This approach, deriving timing from an adjacent cylinder's normal operation, eliminates the need for the secondary system to contain its own timing control.
The most common type of engine brake derives its motion from the injector cam of the same cylinder.
Conventional single-cycle engine braking systems have inherent limitations. These limitations are introduced primarily by (1) secondary valve actuating systems derive there timing from an adjacent cylinder's normal valve opening timing via hydromechanical links; and (2) secondary systems do not interrupt the normal opening and closing of the cylinder intake and exhaust valves during positive power. The first circumstance generally results in a sub-optimum realization of the full engine braking potential. This occurs because the timing and duration of the exhaust valve opening to vent the cylinder at the completion of the compression braking stroke is fixed by an adjacent cylinder's normal timing or injector timing of that cylinder during valve opening duration. The second circumstance prevents exploiting a second compression braking cycle because the exhaust valve is open during the exhaust stroke. Otherwise, the second cycle is available for compression braking. Consequently, a system that takes control of the actuation of the cylinder intake and exhaust valves enables or disables their opening. This can optimize engine performance in an engine braking mode.
Other internal combustion engine limitations have emerged in the thirty years since engine braking technology has been introduced Emission controls, turbo-chargers, and exhaust braking have affected the performance of engine braking. The net effect is a reduction in conventional engine braking performance, particularly at low speeds when the turbo-charged air volume, available for compression, is small. During the same time, demand and reliance on conventional engine braking has increased. A further motivation for improved engine braking performance has emerged.
Engine retarders of the compression release-type are well-known in the art. Engine retarders are designed to convert, at least temporarily, an internal combustion engine of either the spark-ignition or compression-ignition type into an air compressor. In doing so, the engine develops retarding horsepower to help slow the engine down. This can provide the operator increased control over the vehicle, and substantially reduce wear on the service brakes of the vehicle. A properly designed and adjusted compression release-type engine retarder can develop retarding horsepower that is a substantial portion of the operating horsepower developed by the engine on positive power.
A compression release-type retarder of this type supplements the braking capacity of the primary vehicle wheel braking system. In so doing, it extends substantially the life of the primary (or wheel) braking system of the vehicle. The basic design for a compression release engine retarding system of the type involved with this invention is disclosed in Cummins, U.S. Pat. No. 3,220,392.
The compression release-type engine retarder disclosed in the Cummins '392 patent employs a hydraulic control system. The hydraulic control system of typical compression release-type engine retarders used prior to the present invention engage the valve actuation system of the engine. When the engine is under positive power, the hydraulic control system of a typical compression release engine retarder is disengaged from the valve control system. When compression release-type retarding is desired, the fuel supply is stopped and the hydraulic control system of the compression release brake causes the compression release brake to engage the valve control system of the engine.
Compression release-type engine retarders typically employ a hydraulic system in which a master piston engages the valve control or injector system of the engine. When the retarder is activated, a solenoid valve allows lube oil to fill a hydraulic circuit which actuates the master piston which is hydraulically connected to a slave piston. The motion of the master piston controls the motion of the slave piston, which in turn typically opens the exhaust valve of the internal combustion engine at a point near the end of the compression stroke. In doing so, the work that is done in compressing the intake air cannot be recovered during the subsequent expansion (or power) stroke of the engine. Instead, it is dissipated through the exhaust. By dissipating energy developed from the work done in compressing the intake gases, the compression release-type retarder dissipates energy from the engine, slowing the vehicle down.
The master piston in typical compression release engine retarders of the type known prior to the present invention is typically driven by a push tube that is controlled by the engine camshaft. The force required to open the exhaust valve is transmitted back through the hydraulic system to the push tube and the camshaft. Historically, it has been desirable to minimize modification of the engine, as many compression release-type retarders were installed as after market items. Accordingly, a push tube that otherwise moves at a point in the engine cycle close to the desired time to operate the compression release engine retarder was typically selected for actuating the master piston. In some cases, an exhaust valve push tube associated with another engine cylinder was selected. In yet other cases, it was convenient to use the fuel injector cam lobe or push tube associated with the cylinder that was undergoing the compression event. It is also possible to use an intake valve push tube. Additionally, there are other ways to operate the master piston.
Regardless of the specific actuation means chosen, inherent limits were imposed on operation of the compression release-type retarder based on the allowable loads on the engine. A number of mechanical factors have historically imposed limitations: the temperature of critical engine parts, such as valves; the seating velocity of the valves; push tube loads; cam stress; the power available from the compression release retarder to overcome the instantaneous cylinder pressure at the point of opening and a variety of other factors. Typically, it is desired to open the compression release-type engine retarder as late in the engine cycle as possible. In this way, the engine develops a higher degree of compression, allowing more energy to be dissipated through the compression release retarder. Delaying the opening of the exhaust valve in the compression release event to a point later in the compression stroke, however, also increased substantially the loading placed on critical engine components.
Safety, reliability and environmental demands have pushed the technology of compression release engine retarding significantly over the past 30 years. Compression release retarding systems are typically adapted to a particular engine in order to maximize the retarding horsepower that could be developed, consistent with the mechanical limitations of the engine system. In addition, over the decades during which these improvements were made, compression release-type engine retarders garnered substantial commercial success. Engine manufacturers became more willing to embrace compression release retarding technology. Compression release-type retarders have continued to enjoy substantial and continuing commercial success in the marketplace. Accordingly, engine manufacturers have been more willing to make engine design modifications, in order to accommodate the compression release-type engine retarder, as well as to improve its performance and efficiency.
In addition to these pressures, significant environmental pressures have forced engine manufacturers to explore a variety of new ways to improve the efficiency of their engines. These changes have forced a number of engine modifications. Engines have become smaller and more fuel efficient. Yet, the demands on retarder performance have often increased, requiring the compression release-type engine retarder to generate greater amounts of retarding horsepower under more limiting conditions. A variety of ancillary equipment are currently employed on diesel type engines, including turbo-chargers, silencers, exhaust brakes, waste gate controls, electronic controls, sensors and other collateral apparatus.
Similarly, in an effort to secure greater performance, an engine may have a turbocharger. Another method of vehicle engine retarding has included the use of any device that causes a restriction in the turbo, or in which a restriction is imposed in the exhaust manifold, increasing the back pressure on the engine and making it harder for the piston to force gases out of the cylinder on the exhaust stroke. During the past decades many engine manufacturers, and operators, have used an exhaust restriction method on a turbo-charged engine in combination with a compression release-type retarder. The use of the exhaust restrictor, however, essentially "kills" the boost available from the turbo-charger, dramatically reducing the amount of air delivered to the engine on intake. This, in turn dramatically worsens compression release-type engine brake performance. Combination braking does result in an overall increase in retarding due to the practical effect of getting more air into the cylinder.
As the market for compression release-type engine retarders has developed and matured, these multiple factors have pushed the direction of technological development toward a number of goals: securing higher retarding horsepower from the compression release retarder, increasing mid-range performance and variable retarding capability; working with, in some cases, lower masses of air deliverable to the cylinders through the intake system; and the inter-relation of various collateral or ancillary equipment, such as: turbo-chargers; and exhaust brakes. In addition, as the market for compression release engine retarders has matured and moved from the after-market to original equipment manufacturers, engine manufacturers have shown an increased willingness to make design modifications to their engines that would increase the performance and reliability, and broaden the operating parameters, of the compression release-type engine retarder.
In addition, various techniques to improve the efficiency of the engine on positive power --and thereby reduce emissions--have also been incorporated into engines. Among the techniques that have been investigated is the recirculation of a certain portion of the exhaust gases through the engine to attempt to achieve more complete burning of the exhaust gases: exhaust gas recirculation.
Various manufacturers have incorporated exhaust gas recirculation systems into their engines. In some instances, these have been done to achieve exhaust gas recirculation for environmental reasons. In other instances, it has been done to add additional charge to the cylinder that is undergoing the compression release retarding event. Ueno, Japanese laid open Patent Publication No. Sho 63.backslash.1988-25330 (published Feb. 2, 1988), for Exhaust Brake Equipment for Internal Combustion Engine specifically discloses adding an additional cam lobe to open an exhaust valve at the end of the intake stroke or the starting part of the compression stroke. The engine described by Ueno also is equipped with an exhaust brake so that the back pressure in the exhaust manifold is significantly higher than the pressure in the cylinder. At that point, the exhaust gas recirculation event occurs forcing valve opening at the end of intake and/or beginning of compression. Consequently, higher pressure exhaust air from the exhaust manifold flows into the cylinder, increasing the amount of air in the cylinder during the succeeding compression stroke. The greater amount of gas in the cylinder at the beginning of the compression stroke generates increased retarding horsepower.
Volvo has also employed exhaust gas recirculation. Gobert et al., U.S. Pat. No. 5.146,890 for Method and a Device for Engine Braking a Four Stroke Internal Combustion Engine, discloses the addition of an exhaust gas recirculation lobe on the cam. The engine has for each cylinder at least one inlet valve and at least one exhaust valve for controlling communication between a combustion chamber in the cylinder and an inlet system and an exhaust system, respectively. The arrangement also establishes communication between the combustion chamber and the exhaust system in conjunction with the exhaust stroke and also when the piston is located in the proximity of its bottom-dead-center position after the inlet stroke and during the latter part of the compression stroke and during at least part of the expansion stroke. Communication of the combustion chamber with the exhaust system is effected upstream of a throttling device provided in the exhaust system, this throttling device being operative to throttle at least a part of the flow through the exhaust system during an engine braking operation, therewith to increase the pressure upstream of the throttling device. The exhaust gas recirculation lobe on the Volvo cam, however, is at a different cam timing than the exhaust gas recirculation of the present invention. Moreover, nothing in the Volvo '890 patent teaches or suggests two-cycle braking.
In a typical four-stroke internal combustion engine, the intake rocker arm and exhaust rocker arms have dedicated cam lobes. Historically, engine manufacturers have been reluctant to modify their engine configurations to provide a dedicated cam lobe for the compression release-type brake. In addition, on fuel injected engines, the fuel injector requires additional space on the cam shaft for the fuel injector cam lobe. This configuration has historically limited the amount ot space available to provide additional cams to actuate the compression release brake system. The availability of a dedicated cam for the compression release brake system would simplify and improve the operation, reliability, and performance of the compression release-type braking system. Insufficient space has typically been available on the cam shaft, however, to accomplish that objective.
Recently, some manufacturer have begun manufacturing engines with two overhead cam shafts. This provides a greater overall amount of space along the cam shaft to use cams to directly actuate engine components. For example, one engine manufacturer has recently adopted a dual overhead cam shaft design. In the new engine, the fuel injector cam is located on a separate cam shaft, to provide a greater contact length along the cam to operate the fuel injector. This frees additional space along the second valve actuation cam shaft to provide cams that are dedicated to the operation of the compression release-type brake. It is in this type of situation that the present invention has particular application. As embodied herein, the present invention uses a dedicated cam to directly actuate a rocker arm for the compression release-type engine retarder, thereby eliminating push tubes and other associated hardware. This simplifies installation and maintenance of the brake and improves its reliability by reducing the number of parts that are susceptible to failure and, in particular, particularly high stress parts such as push tubes.
In addition, some engine manufacturers have attempted to redesign the overhead of the engine to employ a dedicated compression brake cam. For example, certain model engines feature overhead cam shafts. Engine manufacturers have redesigned the overhead of certain of its engine models to incorporate a dedicated brake cam compression release. For example Vittorio. U.S. Pat. No. 5,586,531 assigned to Cummins Engine Company discloses an engine retarder cycle for an engine in which the exhaust valve is opened earlier during the compression stroke than previously contemplated. Vittorio discloses beginning the ripening of a retarder valve in an engine cylinder during a second half of a compression stroke of a piston in the engine cylinder. By opening the retarder valve earlier, the cylinder pressure is not allowed to build to as high a level as previously attained. The retarder valve is opened to a maximum displacement prior to a top dead center position of the piston. The retarder valve is then closed during the first half of the expansion stroke of the piston. Reedy et al., U.S. Pat. No. 5,626,116, assigned to Cummins Engine Company discloses a dedicated rocker lever and cam assembly for a compression braking system. The Reedy dedicated rocker lever and cam assembly operates according to the method described in the Vittorio '531 patent. The braking system includes an independent exhaust valve actuator assembly having a braking mode rocker lever and a cam lobe for imparting movement to the exhaust valve when the engine is operated in the braking mode.
The present invention is a significant improvement on this type of design. The present invention uses the dedicated cam lobe to effect two-cycle braking and exhaust gas recirculation, in order to provide additional retarding power from the engine. The above-described method and device do not anticipate two-cycle braking.
Sickler, U.S. Pat. No. 4,572,114 is one example of an early effort to develop a fully integrated, high performance, two-cycle compression release-type brake. Sickler's '114 patent discloses a process and apparatus for the compression release retarding of a multi-cylinder four cycle internal combustion engine. The process provides a compression release event for each cylinder during each revolution of the engine crankshaft in which the normal motion of the exhaust and intake valves is inhibited and the exhaust valves are opened briefly at each time the engine piston approaches the top dead center position. The intake valves are opened after each opening of the exhaust valves. The apparatus includes a hydraulic assembly driven by the engine push-tubes which produces a timed hydraulic pulse adapted to open the exhaust and intake valves at the proper time. Hydraulically actuated means are provided to disable the valve crosshead or rocker arm so as to inhibit the normal motion of the valves. The process and apparatus disclosed by Sickler is too involved and has not been commercially developed.
Another method that has been employed to attempt to achieve greater efficiency and performance from compression release engine braking systems is to attempt to achieve "two-cycle" engine braking. Essentially, the engine brake in a typical compression release-type engine retarder operates on only one stroke of a four-stroke engine, namely, at the end of the compression stroke near top dead center. It has long been theorized that greater braking performance could be achieved by attempting to initiate two compression release events per engine cycle during braking operation. Attempts have been made to do so but none of those attempts has yet to produce a commercially viable engine braking system that achieves increased performance. These devices, however, were too complicated with high manufacturing costs and low reliability. Furthermore, the others have not taken their development efforts far enough to develop technology for an engagement device for an overhead cam engine.
One of the principle limitations in achieving effective two-cycle engine braking occurs with a cam shaft operated valve train in a four-cycle engine. The normal exhaust valve motion must be disabled in order to retain the gases in the cylinder and achieve braking on a second stroke of the engine, when opening the exhaust valve before the second TDC which is the normal exhaust stroke TDC. Prior to this, new air has to be admitted to the cylinders before the second compression release event occurs. Otherwise, the air simply exits through the exhaust valve on the exhaust stroke. The ability to add a second cylinder fill event prior to the second braking event is also challenging. No prior engine braking systems of which the present inventors are aware have been able to overcome these two limitations and achieve an effective second braking event.
None of these methods, however, provide solutions to certain of the problems of compression release-type retarding. First, none of these prior systems disclose, teach, or suggest how to achieve reliable, effective two-cycle braking while actuating the valves, namely, without using a "bleeder" type brake. Second, none discloses, teaches, or suggests how to optimize the actuation of the exhaust valve during the intake and compression strokes in order to achieve the highest possible retarding horsepower from the compression release event without exceeding the mechanical limits of the engine. In addition, none of these methods discloses, teaches or suggests any method for the use of exhaust gas recirculation to regulate the exhaust pressure in the exhaust manifold least of all in the context of two-cycle braking.
Prior compression release-type brakes are typically optimized at the rated speed of the engine. The engine, however, is not always operated at its rated speed and, in fact, is frequently operated at significantly lower speeds. The advertised retarding performance based on the rated speed cannot be achieved when operating at lower engine speeds called mid range. It is therefore highly desirable to provide a method for controlling the braking systems and better tuning them to the speed at which the engine is operating. This is not possible with most prior methods, including those discussed above.
There remains a significant need for a method for controlling the actuation of the exhaust valves in order to increase the effectiveness of and optimize the compression release engine retarding. Further, there also remains a significant need for a system that is able to perform that function over a wide range of engine operating parameters and conditions. In particular, there remains a need to "tune" the compression release-type retarder system in order to optimize its performance at lower operating speeds than the rated speed of the engine.
In spite of the existence of the substantial incentives and prior work to develop effective two-cycle braking, none of the known efforts to do so have been successful. There remains a significant need for an effective two-cycle braking system that provides greater increased retarding power. In addition, providing effective two-cycle braking essentially requires assuming control of the valves from the valve train over a greater range of the engine braking cycle. There remains a significant need in the field for the invention to achieve this valve control. Again, however, in spite of the substantial need for these systems, no effective systems have been able to produce this valve control, let alone in both positive power and engine braking operation.
The present invention describes a process and apparatus that accomplishes both goals. It enables effective two-cycle braking to occur. The present invention is usable in multi-cylinder engines having one or more intake valves and one or more exhaust valves per cylinder. The present invention achieves essentially two-cycle engine braking and is capable of assuming control of valve actuation in both positive power and engine braking operation.